1. Field of the Invention
This invention relates to a control device for a travelling wave type motor, and particularly to a control device for controlling the driving of a travelling wave type motor so as to be able to effect high-speed positioning and precise positioning of the motor.
2. Related Background Art
An example of the basic construction of a travelling wave type motor is comprised of two groups of piezo-electric elements having, for example, a positional phase difference of .lambda./4 therebetween and secured to a vibration resilient member formed into a circular ring-like shape, a vibrator for forming a travelling wave by the combination of two standing waves excited by AC fields having a time phase difference of 90.degree. therebetween being applied to said two groups of piezo-electric elements, and a moving member brought into pressure contact with the travelling wave formed on a surface of said vibrator and driven by the frictional force with the travelling wave formed on a surface.
Also, the travelling wave type motor, because of its characteristics that the stability of low-speed rotation is good and that a high torque can be output, has the feature that it can directly drive a driven member without using an output transmitting mechanism such as a gear mechanism.
On the other hand, in a travelling wave type motor, three factors generally determine the parameters which make the speed variable, i.e., the amounts of operation; the amplitude value of the vibration of a stator, the driving frequency and the phase difference between two AC fields. These factors will hereinafter be referred to as the amplitude operation, the frequency operation and the phase difference operation.
Description will hereinafter be made of the problems when the amounts of operation of these three factors are individually varied.
(1) The amplitude operation is accurate to make the absolute value of the speed variable, but cannot operate the direction of the speed.
(2) In the frequency operation, as in the amplitude operation, the direction of the speed, i.e., the direction of movement, cannot be operated. Also, the travelling wave type motor utilizes the resonance characteristic of a vibration member and therefore, the operation of vibration amplitude by taking off the optimum frequency which is one of the natural frequencies is an operation which reduces driving efficiency, and is not preferable.
Further, the frequency characteristic of vibration, as shown in FIG. 3A of the accompanying drawings, cannot always be said to be of a smooth single peak property, and often is a degenerative characteristic as shown in FIG. 3B of the accompanying drawings wherein multi peaks overlap one another in a narrow band, or a characteristic as shown in FIG. 3C of the accompanying drawings which is liable to cause a "precipitous drop" phenomenon in which the right or left gradient becomes sharp in spite of a single peak property, and a desired speed has been difficult to obtain.
(3) In the phase difference operation, the phase difference between two phases is changed from -90.degree. to 90.degree., and in principle, the speed can be operated as a vector including that direction.
However, the generation of a travelling wave is genuinely established at the phase differences .+-.90.degree., and at an intermediate value thereof, it allows the generation of an unnecessary standing wave and thus, has reduced driving efficiency. This standing wave is also a factor which damages the frictional material between a stator and a rotor and aggravates the durability of the motor.
From the above-noted points, an amplitude command is preferable as the amount of operation of the absolute speed value, and to change the direction of the speed, it is necessary to use a phase difference command in some way.
FIG. 2 of the accompanying drawings shows a block diagram of a control device according to the prior art. The reference numeral 100 designates a control calculator which outputs an amplitude command (torque strength) value and a rotation direction command value on the basis of the detected positional data and an input positional command. The reference numeral 300 denotes a driving circuit in which there are prepared an amplitude command input terminal and a phase difference changeover switch and to which are input the amplitude command value output from the control calculator 100 and the information of the rotation direction for changing over the phase difference.
In the driving circuit 300, the changeover of the phase difference is realized by reversing the waveform of one phase. These two control inputs correspond to the designation of the absolute speed value and the selection of the speed direction, and are subjects which cannot be handled as linear elements.
The operation of this control device is such that when a target position is first input to the control calculator 100, the operation direction is defined by the relation with the current position of the rotor of a motor 7 and a rotation direction command value is output to the driving circuit 300, which thus operates the phase changeover switch.
Next, the control calculator 100 calculates a control error, and outputs an amplitude operation command to the driving circuit so as to accelerate, fix and decelerate the speed in accordance with a speed profile such as a trapezoid.
The cycle of the operation pattern comprises the setting of the operation direction as an initial driving value, and the starting and stoppage of the operation in one direction, and if required, this cycle is repeated. As described above, in the control device according to the prior art, the control system is a control system comprising chiefly a sequence operation, and it has been difficult to use the travelling wave type motor as a sensitive servo system.
On the other hand, as another feature of the travelling wave type motor, mention may be made of the fact that a great holding torque is provided by a frictional force created by the moving member (hereinafter referred to as the rotor) being always in pressure contact with the vibrator (hereinafter referred to as the stator). In order to make the most of this feature, in the positioning of the travelling wave type motor, the operation of making the voltage given to the motor zero is performed after the termination of the positioning operation. This operation is readily realized by cutting off the electrical signal path to the motor and grounding the input end to the motor.
Now, the travelling wave type motor having the above-described feature has a brake function which is not possessed by an electromagnetic motor or a pulse motor and therefore, even if the electrical signal path to the motor is cut off when the positioning is terminated, that is, when the angular speed of the motor becomes zero and the position of the motor coincides with the target position, the travelling wave type motor can be said to potentially have a positioning operation ability much more excellent than that of the electromagnetic motor or the pulse motor.
However, if the electrical signal path is cut off before the angular speed of the motor becomes zero, the motor will be stopped in a short time by the frictional force between the rotor and the stator, but the motor will be rotated beyond the target position by its inertia force and therefore, may cause a reduction in positioning accuracy. If the angular speed is sufficiently small, the motor will be stopped in a moment by such frictional force and no position error will arise, but in any case, it is necessary to wait for the angular speed to become sufficiently small and therefore, it has been impossible to shorten the positioning time.
That is, as shown in FIG. 7 of the accompanying drawings, the positioning to the target position value r is such that the motor is accelerated until a time t.sub.7, keeps a constant speed from the time t.sub.7 until a time t.sub.8, is decelerated after the time t.sub.8 and is stopped at a time t.sub.9. This locus on phase plane is shown in FIG. 8 of the accompanying drawings. In FIG. 8, the abscissa represents deviation e and the ordinate represents the differentiated value (e') of the deviation e.
On the other hand, it is often the case that the positioning operation is repeatedly performed, and the characteristic of the travelling wave type motor driven by frictional force is varied by a temperature change which is necessarily caused by the positioning operation being repeated and therefore, it has been difficult to obtain a stable positioning performance.
Also, due to problems in the working accuracy of a stator during the manufacture of the motor, the characteristic of the motor also differs by the stop angle of the rotor and therefore, if has been impossible to obtain a constant performance over the full periphery of the motor.
On the other hand, where the control device for the travelling wave type motor is grasped as a circuit element, in the output stage construction as shown, for example, in FIG. 10 of the accompanying drawings wherein electrical vibrations of two phases power-amplified by a pair of power amplifiers 200 are applied to the travelling wave type motor, the reference numeral 400 designates a driver comprising chiefly analog circuits such as an oscillation circuit and an amplitude modulating circuit, and a portion which performs an operation of high intelligence such as position control loop compensation is of a construction which shares the role with a microprocessor 500.
The function of the driver 400 basically is to electrically create ultrasonic vibrations of two phases. However, the conditions of two-phase output vibrations are:
1) That they are predetermined ultrasonic range frequencies; PA1 2) That the frequencies of two phases are completely coincident with each other; PA1 3) That they are equal to each other at a predetermined vibration amplitude; PA1 4) That they have a predetermined phase difference therebetween; and PA1 5) That they are sine wave vibrations free of strain.
Here, assuming, for example, pulse wave driving which does not impose condition 5) above, it can be relatively easily realized by the digital circuit technique to satisfy all the above conditions.
However, the vibrator of the travelling wave type motor generally has numerous harmonic wave resonance modes and thus, excites an unnecessary resonance mode for a pulse wave or a distorted vibration wave, and it is difficult to produce a uniform travelling wave. To obtain a driving force of good quality, it is a requisite condition for the two phases of the driving voltage to be genuine sine waves and satisfy an accurate synchronous relation.
In the prior art, a driving circuit which attempts to satisfy conditions 1) to 5) above has been constructed with chiefly analog elements as hardware, inclusive of digital elements.
However, the prior-art control device has been an elaborate circuit, but nevertheless could not be expected to be of high accuracy. This comes from a general problem peculiar to analog circuits. Here, the condition of the coincidence of a driving signal with piezo-electric elements of two phases can be satisfied at least steadily by installing an appropriate compensation loop, but as regards the condition of the absolute value such as the resonance frequency, it is difficult to enhance accuracy when the thermal drift of circuit elements is taken into account.
Also, to give the initial setting which takes the individual differences between travelling wave type motor bodies into account to such a driving circuit, a large cost and labor for regulation are required, and this has been a reason for the difficulty of mass production.
Further, it is desirable that the "predetermined values" of the frequency, amplitude and phase difference in the foregoing conditions can be arbitrarily changed as the amounts of operation of the travelling wave type motor, but it has been difficult to input all these amounts of operation to the circuitry of the hardware and process the same.
That is, the prior-art control device is of a type in which, for example, only an amplitude input is prepared in the driver 400 and is connected to the microprocessor 500 to thereby effect speed adjustment, and has been contented with a control system construction which denies the degree of freedom of operation.
Problems the present invention intends to solve are concerned with the following three points.
Firstly, the fundamental problem peculiar to the travelling wave type motor is that originally, speed is a vector amount having the speed and the direction at a time, nevertheless the travelling wave type motor is of physical structure in which two state amounts, i.e., the speed and the direction, are separated from each other.
This point indeed is the reason why the travelling wave type motor has been a non-linear element difficult to handle, and is a problem left to be solved. Further, the following problem is posed in practical use. As described above, in the prior art, it has not been taken into consideration to rotate the travelling wave type motor continuously in forward and reverse directions, and the prior-art travelling wave motor has been approximate to sequence control and could not assume a complete feedback control construction.
Where such prior art is used for positioning control, when overshoot occurs, the processes of stoppage, judgment and direction changeover come in the course of positioning, and this has been very disadvantageous from the point of the evaluation of the positioning time. Further, near the target point, there is an operation delay based on such a sequence operation and therefore, when the gain is high, hunting has remained, and when the gain is low, control deviation has remained.
Thus, in the prior art, accuracy and response speed could not be improved and the travelling wave type motor has been regarded as an actuator of low positioning performance. Also, in DC and AC motors which are other type actuators, the linearity between the amount of operation and the amount of control are good and various servo techniques such as classical and modern control theories can be applied to such motors, whereas the application of these powerful techniques to the prior-art travelling wave type motor has been impossible. The prior-art travelling wave type motor has been regarded as a special servo technique which is not in harmony with the linear control theory, and this has been the cause of the travelling wave type motor being unpopular among users.
A second problem is concerned with means for improving the response performance of positioning control, and is that there has not been discovered or practised appropriate means for effectively utilize the braking function of the travelling wave type motor itself.
A third problem is that the vibrator of the travelling wave type motor must be strictly resonated and there are many parameters regarding resonance and therefore, highly accurate sine wave vibration could not be generated and as a result, a uniform drive force could not be obtained, that the degree of freedom of driving could be realized only in a limited form and that hardware regulation was difficult.